1. Technical Field
The present invention relates to optical devices, and in particular, planar optical waveguides.
2. Art Background
Optical fiber communication systems are becoming more prevalent. In addition to the optical fiber itself, optical fiber communication systems use a wide variety of optical devices for receiving, transmitting, and using optical signals. One type of integrated optical device is a silica optical circuit formed on silicon substrates. The basic structure of such devices is described in Henry, C. H., et al., "Silica-based optical integrated circuits," IEE Proc.-Optoelectron, Vol. 143, No. 5, pp. 263-280 (1996). The waveguide is formed from 3 layers; those layers are a lower cladding (referred to as a base in Henry) core and upper cladding. The lower cladding layer isolates the fundamental mode from the silicon substrate. Such isolation prevents optical signal leakage through the silica-silicon (substrate) interface, which, unlike other waveguide interfaces, is not totally reflecting. The refractive index of the upper cladding layer is chosen to be nearly equal to that of the base layer.
The lower cladding layer is made out of undoped or lightly doped silica. This is the most rigid layer and it keeps the core, which is adhered to it, from moving after it is patterned. The other glasses are more highly doped silica.
A variety of processing strategies for fabricating silica-on-silicon, optical devices have been proposed. Such processing strategies typically require that the base layer be deposited on the surface of the silicon substrate. Current techniques for forming the lower cladding are deposition processes such as low-pressure chemical vapor deposition (LPCVD) and high pressure oxidation (HiPOX). While such deposition techniques provide an acceptable lower cladding, these techniques are somewhat slow. Accordingly, alternative techniques for forming the lower cladding for silica-on-silicon optical devices are sought.